types: make _id required on Document type

[vkarpov15]

Fix #14660

Summary

_id: T is more correct because _id should always exist on a hydrated document by default. And this doesn't break any tests other than ones that assert on the behavior from #14660. @hasezoey what do you think, does this work fine with Typegoose?

Examples

[hasezoey]

LGTM, works in typegoose, likely because typegoose already always did a mongoose.Document & mongoose.RequireId for typegoose's DocumentType (though it could not be removed because of the inference of _id as ObjectId if not explicitly defined)